Clinical jaundice is the most common medical diagnosis in the newborn; over 50 percent of neonates display significant unconjugated hyperbilirubinemia during the first 5 days of life. Phototherapy is the most common treatment; hundreds of thousands of babies in the U.S. are irradiated each year with white or blue light to decrease plasma levels of unconjugated bilirubin and reduce the risk of permanent brain damage. The broad, long-term objectives of the project are to understand bilirubin structure-function relationships and the molecular basis of bilirubin detoxification; to define the interdependence of 3-dimensional structure, hydrogen bonding, hydrophobicity and acidity in hepatobiliary uptake, glucuronidation and excretion; to elucidate fully the molecular mechanism of phototherapy. The SPECIFIC AIMS are to: 1. Determine the 3-dimensional structure of bilirubin, its anions, salts, conjugates and photoisomers in biomimetic and biological environments, including human serum albumin, by molecular dynamics and spectroscopic methods. 2. Design and prepare bilirubin analogs of known chirality and varying shape, with substituents of differing size and polarity to understand physicochemical properties important in metabolic and photobiologic mechanisms. 3. Clarify the interdependence of bilirubin acidity (pKa) and hydrogen bonding in amphiphilicity and in the potential role of bilirubin as an ionophore and ion transporter. 4. Characterize new bilirubin photoreactions, their action spectra, and their importance to phototherapy. 5. Elucidate the mechanism of bilirubin photocyclization and its stereochemistry, and investigate the influence of local environment including protein binding on the channeling of bilirubin photochemical reaction pathways. 6. Synthesize chemically-designed bilirubins as molecular probes of the interrelated roles of pigment stereochemistry and intramolecular hydrogen bonding, hydrophobicity and acidity, vinyl substitution and polarity in hepatic uptake, conjugation and excretion. These studies are directly relevant to the prevention of brain damage in the jaundiced newborn, to improving our understanding of bilirubin metabolism and liver disease. They are designed to clarify how bilirubin molecular structure controls its hepatic processing and elimination and to lead to improved and safer methods for treatment of severe unconjugated hyperbilirubinemia in the neonate and in patients with Crigler-Najjar syndrome.